The period derivatives for 56 pulsars have been obtained from two sets of timing observations 1 yr apart. A total of 84 period derivatives are now known. Statistical relationships between period, period derivative, magnetic field strength, luminosity, integrated pulse width and apparent age are examined. The distribution of values of period and period derivative is explained by postulating a secular decay of magnetic dipole moment. The cut-off of radio emission appears to be related to the magnetic field near the velocity of light cylinder.